1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that forms registration patches of different colors on an intermediary transfer member and detects the thus formed registration patches of the different colors to correct for a color displacement in a color image based on the results of the detection.
2. Description of Related Art
For example, in a tandem-type color image forming apparatus, image forming units for different colors, each built around a photoconductive drum and a transferring device, are arranged along an intermediary transfer belt. In each image forming unit, the photoconductive drum is scanned with, and exposed to, a laser beam from a laser diode to form an electrostatic latent image, which is then developed with toner to form a toner image. Thereafter, those toner images are primarily transferred to the intermediary transfer belt in a superposed fashion. The toner images of the different colors thus primarily transferred are then, at a secondary transfer position, secondarily transferred, all at once, to a sheet of a printing medium that passes between the intermediary transfer belt and a secondary transfer roller which rotates in contact with it.
In such a color image forming apparatus, factors such as secular deterioration of the photoconductive drum, intermediary transfer belt, secondary transfer roller, etc. and variations in temperature and humidity around the apparatus can cause, among others, a displacement between the positions exposed to the laser beam for the different colors, resulting in a color displacement. To prevent that, color displacement correction is often performed. One example of color displacement correction is registration correction, whereby registration patches comprising toner images of different colors are formed on the intermediary transfer belt and, based on the positions at which the registration patches of the different colors are formed, the displacement amounts of the different colors are detected, and the image write positions are corrected.
During registration correction, if the secondary transfer roller is in contact with the intermediary transfer belt, the registration patches on the intermediary transfer belt attach to the secondary transfer roller and soil the roller surface. The soil on the roller surface then, during image formation, soils the reverse side of the sheet. To prevent that, conventionally, the secondary transfer roller is configured to be releasably contactable with the intermediary transfer belt and, during registration correction, the secondary transfer roller is so controlled as to be released from the intermediary transfer belt.
However, as the coefficient of friction between the intermediary transfer belt and a driving roller that makes it rotate decreases with use, a difference arises in the rotation speed of the intermediary transfer belt between when the secondary transfer roller is contacted with the intermediary transfer belt and when the secondary transfer roller is released from the intermediary transfer belt. Thus, actual image formation and registration correction are performed under different conditions, and this makes accurate correction of a color displacement impossible. One possible solution is to increase the belt tension of the intermediary transfer belt to make the speed of the intermediary transfer belt constant irrespective of whether the secondary transfer roller is contacted with or released from the intermediary transfer belt. Increasing the belt tension, however, can lead to, among others, a crack in the intermediary transfer belt.
Thus, for example, Patent Document 1 (Japanese Patent Application Publication No. 2009-47741) discloses a technology whereby registration patches are formed with the secondary transfer roller both contacted with and released from the intermediary transfer belt and the difference in the rotation speed of the intermediary transfer belt depending on whether the secondary transfer roller is contacted with or released from the intermediary transfer belt is calculated to correct displacement amounts.
However, with the technology disclosed in Patent Document 1 (Japanese Patent Application Publication No. 2009-47741), the registration patches need to be formed also with the secondary transfer roller contacted with the intermediary transfer belt, and registration correction requires more time and toner.
Against the background discussed above, an object of the present invention is to provide an image forming apparatus that can perform registration correction accurately without requiring too much time or toner, with consideration given to the difference in the rotation speed of an intermediary transfer member depending on whether a secondary transfer roller is contacted with or released from the intermediary transfer member.